It is missing in the preview in Jalbum only, the resulting album seems to be fine.
Tried with the setting on and off, the preview keeps not displaying the title, the album on disk shows fine (the nice thing about Lucid is that local preview with Chrome etc just works. So set in Tools - Preferences - Preview to external browser to work around the problem for now.
Some caching problem in the internal web browser of Jalbum?
I see now: it doesn't affect Chrome though, which makes the favor of displaying the container as flexbox, provided the flex-flow attribute is specified. FireFox follows the standards and won't use it as flexbox, just don't want to guess the author's intent.